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How trees become elders in a human-dominated world

DAILY SCIENCE

How trees become elders in a human-dominated world

A team of researchers combed through a database of 1.8 million century-old trees in China to solve an Anthropocene puzzle
May 16, 2023

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Old trees are ecologically important and have been revered and protected by people across cultures and throughout history. But old trees also get in the way of human activities, such as logging, agriculture, and other development. Despite increasing interest in conserving these “elderflora,” we don’t know much about what enables old trees to survive nowadays, especially in human-dominated landscapes.

To find out more about this question, researchers assembled a database of nearly 1.8 million individual century-old trees found in human-dominated landscapes throughout China. The trees represented 1,580 of the roughly 3,500 tree species that are found in the country.

The researchers then developed two new metrics to help understand the distribution and commonness of old trees of different species. First, they calculated what proportion of a given tree species’ range had old trees of that species in it. Second, they calculated what proportion of the old-tree species inhabiting a particular location were represented by old trees there. They also analyzed the relationship between these two metrics and various biological, environmental, and cultural factors.

Just a handful of species account for most of the old trees in China, the researchers report in the journal Nature Plants. “Three species were represented by over 100,000 old trees each, and 30 species had over 10,000 old trees each. In contrast, 653 species were represented by fewer than 10 trees apiece, and 264 species had only one individual,” they write.

 

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Tree species with many old individuals spread across a wide range included the ginkgo tree (Ginkgo biloba), Chinese pistache (Pistacia chinensis), and camphor tree (Cinnamomum camphora). Half of the old-tree species were found as old trees in less than 20% of their potential range.

Not surprisingly, the species most likely to persist past the century mark in human-dominated environments tended to be ones that humans make use of in some way – as medicine, food for people or livestock, valuable timber, ornamental plantings, or for their religious or other cultural value.

“At the national scale, human-associated species accounted for only 14.6% of the total number of old-tree species but 86.7% of the total number of old trees,” the researchers write. “In short, the occurrence of old trees in most regions was dominated by a suite of common human-associated species, analogous to bio-homogenization emerging in modern times in many places.”

Old trees in intensively cultivated areas were especially likely to belong to such human-associated species, the researchers found. But old trees belonging to other species that aren’t utilized or cultivated by humans could be found in mountainous areas and areas inhabited by ethnic minorities.

Biological attributes come into play, too: tree species with greater potential height had a greater chance of hitting the century mark, consistent with the known pattern of taller tree species being longer lived. Trees with smaller leaves – especially conifers in the cypress and pine families – also were disproportionately represented among old trees; these groups are known to be relatively long lived and tolerant of varying environmental conditions.

Conservation efforts for old trees should prioritize the species that are represented by rarer and more sporadic old trees, the researchers argue. Climate and terrain may play a role in helping old individuals of such species persist in remote areas. But cultural practices of traditional villages likely contribute as well. Tree species that aren’t directly useful to humans “have often been protected collectively in fengshui forests or individually as sacred trees,” the researchers write. Such practices could serve as a model for protecting a more diverse suite of elderflora elsewhere, they say.

Source: Huang L. et al.  “Human activities and species biological traits drive the long-term persistence of old trees in human-dominated landscapes.” Nature Plants 2023.

Image: hp koch via Unsplash.

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